The tornado season of 2008: climate change to blame? And, tropical update

Residents of Parkersburg, Iowa continue to assess damage and clean up from the tornado that killed six people on Sunday. The tornado was rated EF-5, the highest possible rating for a tornado. An EF-3 tornado also hit Hugo, Minnesota on Sunday, killing one person. Only five new tornado reports occurred yesterday, and severe weather is expected to remain relatively low for the next two days. A new storm system is expected to bring an enhanced chance of severe weather to the upper Midwest beginning Thursday. The deaths Sunday push this year's tornado death toll to 110. This makes 2008 the 12th deadliest tornado season since 1950, and the deadliest since 1998, when 130 deaths were recorded. Assuming that the Parkersburg, Iowa tornado was an EF-4 or EF-5, there have been nine violent EF-4 or EF-5 tornadoes this year. This is the most since 1999, when 13 such twisters were recorded. The total (preliminary) number of tornadoes so far this year is 1191. I doubt that we will break the all time record of 1817 tornadoes in a year, set in 2004, but 2008 may vault into second place if we can top 1998's 1424 tornadoes. Could this year's tornadoes be a sign of climate change?

Figure 1. Tornadoes deaths in the U.S. by year since 1950. Year 2008 deaths are as of May 26.

Well, let's be clear that human-caused climate change is occurring, and will significantly affect nearly all aspects of weather and climate in the decades to come. However, many of these changes will be so small or gradual that they will not become detectable until many decades hence, since there is a large natural variability in weather. As I noted in my February blog, Are tornadoes getting stronger and more frequent?, there is new research that predicts that we may see an increase in the severe thunderstorms that spawn tornadoes by the end of the century. However, the computer modeling efforts that predict this rise in severe weather are just beginning, and much more research remains to be done before we can believe these preliminary results.

Will we be able to detect changes in tornado frequency if they occur?We won't be able to detect changes in tornado frequency due to climate change, unless there is a very large change. We need a technology that can detect all tornadoes, all the time in order to be able to evaluate changes in tornado frequency. Doppler radar can only "see" perhaps 50% of all tornadoes, and many of those it detects never touch down. Thus, we rely on human observers to spot tornadoes, or look for buildings that got in the way of a tornado, using the damage pattern to identify a tornado. If there are no humans around to see a tornado, and if a tornado does not encounter any structures, it will go unrecorded. As the population increases and more buildings are erected, tornado reports will increase. This factor alone can account for the observed increase in total tornadoes since 1950 (Figure 2).

Is there evidence that strong and violent tornadoes are increasing?Strong tornadoes (EF2 and EF3 on the Enhanced Fujita Scale) and violent tornadoes (EF4 and EF5, or F4 and F5 on the pre-2007 Fujita Scale), which make up less than 25% of all tornadoes, cause a large fraction of the tornado deaths. These storms are less likely to go uncounted, since they tend to cause significant damage along a long track. Thus, the climatology of strong and violent tornadoes may offer a clue as to how climate change may be affecting severe weather. Unfortunately, we cannot measure the wind speeds of a tornado directly, except in very rare cases when researchers happen to be present with sophisticated research equipment. Tornadoes are categorized using the Enhanced Fujita (EF) scale, which is based on damage. So, if a strong or violent tornado happens to sweep through empty fields and never destroy any structures, it will not get a rating. Thus, if the number of violent tornadoes has actually remained constant over the years, we should expect to see some increase in these storms over the decades, since more buildings have been erected in the paths of tornadoes.

However, if we look at the statistics of strong and violent U.S. tornadoes since 1950 (Figure 2), there does not appear to be any increase in the number of these storms. In fact, there appears to be a decrease, although the quality of the data base is probably not good enough to say this with confidence. It appears likely that climate change has not caused an increase in the strongest tornadoes in recent decades. I believe we can blame 2008's nasty tornado season on an unusually far south loop that the jet stream has taken this year over the U.S., thanks to natural variability in the weather.

Figure 2. Total, strong and violent tornadoes in the U.S. by year since 1950. The year 2008 (not pictured) has had 128 strong or violent tornadoes as of May 26, according to Wikipedia.

Possible development in the Western Caribbean or Eastern Pacific late this weekA weak low pressure area (Invest 90E) has developed in the Eastern Pacific off the coast of Guatemala, near 10N 90W. This low has the potential to develop into a tropical depression by the end of the week, according to the UKMET model. Other models, such as the GFS, Canadian, and ECMWF, foresee that this area of disturbed weather will not have time to develop before moving northwards over Central America by the end of the week, bringing heavy rains to the region. Once over land, this low might move over the waters of the Western Caribbean and allow a tropical depression to form, as predicted by the GFS model. The NOGAPS model, in contrast, predicts that a tropical depression will form in the Western Caribbean south of Cuba, with no development in the Eastern Pacific. Given the persistence of these computer models over the past week in developing something in the region, I'd put the odds of a tropical depression forming within 7 days at about 40% in the Eastern Pacific, and at 20% in the Western Caribbean. There is a lot of wind shear predicted to prevail near or over the Western Caribbean late this week and early next week, reducing the odds that any such development could hold together long enough to affect the U.S. Regardless, residents of Guatemala, El Salvador, Honduras, Belize, and southern Mexico can expect heavy rains and possible flash flooding late this week from this system.

Figure 3. Area of disturbed weather over the Eastern Pacific that is forecast by some models to develop into a tropical depression. The NHC Graphical Tropical Weather Outlook is a good tool to track this disturbance.

Thanks mlc, couldn't remember, I'm so beat ;o). That would put another upward pulse of the MJO in the area around the beginning of July, when I think TS formation potential would be on the rise for a variety of other reasons.

I think Baha's obs is on the money, too. Much of this convection is associated with the ITCZ and a result of the MJO pulse, too. Yes, there's some rotation, and a low; but, there's lots of converging air in the area from lots of directions. But, you can see some higher tops firing, too. We may get something, but I still don't seeing moving towards the CONUS anytime soon with dry air and searing high pressure.

I am not seeing the same Jp from 2006 and 2007. U sort of like another "eye".

Huh???

What is this, ID theft game or something?

BTW, while a lot of "reglars" are in the blog, PLEASE, my friends, let us keep the troll action to a minimum. We have a few summer season posters who aren't really interested in weather; let's not get ourselves banned because we are not using those BUTTONS. . . . I think this is setting up to be a very interesting season, and it will be much more interesting if we are all here, able to "battle out" the more interesting nuances of each current, past, or future system, potential system, former system, and revitalized, memorialized, homogenized system . . .

865. jphurricane2006 2:18 AM GMT on May 28, 2008 damn my math sucks, either way my point doesnt change, and it better move north or it would run into panama anyway, also how do we know that jump east isnt a relocation, weak systems do that

854. extreme236 2:13 AM GMT on May 28, 2008 Based upon the information I have seen from the computer models and the NHC I don't think 90E will amount to much. I could be wrong but I am still leaning toward development of the Caribbean low.

Good night all!

I am also going more for the development in the Caribbean...anyway i am also out, night all!

CO2 into the atmosphere by man made sources in the United States of America.

Loss of C02 removing forest in less developed countries.

Net effect on CO2 levels hence..

thus and therefore... the larger contribution to green house gas build up and the catostrphic global emergency know as GW. I am actually challenging anyone for a scientific, math based, equation with the US inputs to atmosphere, the mathematical scientific scrubbing capacity of x number of square miles of rainforest.

Annualized.

Comparative.

Hypothesis - deforestation in the third world contributes more to the net gain of CO2 in the atmosphere - by loss of C02 conversion, than the United States of America contributes to the GW problem.

Answers on the 10pm news. No conjecture please... real numbers.

honestly i am just curious about the "hypothesis"================================Source: Global and regional drivers of accelerating CO2 emissions(See supporting data links)Global Summary (2005): In 2005, the cumulative global fossil-fuel emission of CO2 was C(t) = 319 GtC and the cumulative emission from the other major CO2 source, land use change (J. G. Canadell, C.L.Q., M.R.R., C.B.F., E. T. Buitenhuis, et al., unpublished data) was 156 GtC (3). Of the total cumulative emission from both sources (~480 GtC), less than half (~210 GtC) has remained in the atmosphere, the rest having been taken up by land and ocean sinks (4). For the recent period 2000-2005, emission fluxes averaged 7.2 GtC y-1 from fossil fuels and 1.5 GtC y-1 from land use change; through this period the fossil-fuel flux grew rapidly at ~3% y-1, and the land use change flux remained approximately steady. A time-dependent indicator of sink effectiveness is the airborne fraction, the fraction of the total emission flux from fossil fuels and land use change that accumulates in the atmosphere each year. Recent work (J. G. Canadell, C.L.Q., M.R.R., C.B.F., E. T. Buitenhuis, et al., unpublished data)shows that the airborne fraction has averaged 0.44 for the period 1959-2005, increasing slightly through those 47 years to an average of 0.48 for 2000-2005. This implies a slight weakening of land and ocean sinks relative to total emissions.

Evening everybody. It's been a long rough day, but a light rain shower just as I got home had added a good note to the day . . .

Someone commented earlier about the MJO moving into the area, and it's really obvious in the increased convection along the ITCZ and even through this area. Other fronts over the last few weeks stirred up little rain here.

Looking at the models along w/ current conditions, I'm still not sure we'll get even as much as a named system out of that general area of disturbance; even the EPac side will need more time with continued favorable conditions. The upswing in moisture may, however, actually trigger something for the opening of the season.

Anybody has an idea when another moist period of the MJO is supposed to pass through the region? If it's in July, I can reasonably expect at least one tropical system to wind up at that time.